Temperatures are expected to spike Friday as Southern Californians brace for a blistering weekend that has prompted heat advisories and the opening of cooling centers.

The National Weather Service issued heat warnings for large swaths of the state, including parts of Imperial, Kern, Los Angeles, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara and Ventura counties.

Many inland valleys and desert areas could see temperatures well above 100 degrees for the next several days. The mercury could top 120 degrees in the Coachella Valley and 129 in Death Valley, still short of the 134-degree record set there in 1913.

The heat is a particular concern to firefighters because it comes in a year of record dry conditions that have already sparked several major brush fires across Southern California.

On top of that, fireworks go on sale in some areas beginning Friday, adding another fire danger.

Fireworks are to be sold in 295 designated communities in the state through the Fourth of July.

Since January, the California Department of Forestry and Fire Protection has responded to about 2,900 fires, department spokesman Daniel Berlant said. In an average year, he said, it would have responded to fewer than 1,800 by this time.

This increase in fire starts results from the prevalence of dry brush, Berlant said. He added that current weather conditions are more typical of late August or early September.

"We're in a long-term drought," said Bill Patzert, a climatologist with the NASAJet Propulsion Laboratory in La Cañada Flintridge. "The situation is extremely crispy and dry. That equals incendiary."

The high temperatures combined with low humidity and dry brush increase the danger of wildfires, said Bonnie Bartling, a weather specialist at the National Weather Service's Los Angeles station. But because high winds are not forecast for most areas, the service has issued a fire warning only in southern Santa Barbara County.

Somewhat lower temperatures are expected starting Monday, but most residents still will find it plenty hot.

“'Cooler' is a relative term," Bartling said.

Because of the rising heat levels and low humidity, the U.S. Forest Service has extended the hours of its staffing in the Angeles National Forest, effective at least through Monday.

Personnel will be on high alert July 4, even though fireworks are never permitted in the park, spokesman Nathan Judy said.

"If we have fires, the chance of their growing larger is that much greater," Judy said.

The Los Angeles County Fire Department is increasing its staffing Tuesday in anticipation of the holiday, spokesman Tony Akins said. Peak staffing of about 90 firefighters is scheduled for the Fourth of July.

The heat wave could be especially dangerous for the elderly, small children and people with chronic ailments, authorities said.

Several agencies opened cooling centers — air-conditioned public facilities that can be used to escape the heat. Information about the centers can be found by dialing 211, the county's information line. Click on the following link to find an interactive map of the centers.